This invention relates to the field of geophysical prospecting. More particularly, the invention describes methods for use in electroseismic exploration utilizing nonlinear electroseismic conversion mechanisms.
The electroseismic method is a geophysical prospecting tool aimed at creating images of subsurface formations using conversions between electromagnetic and seismic energy, as described in U.S. Pat. No. 5,877,995 (Thompson and Gist). The essence of the electroseismic method is that high levels of electrical energy are transmitted into the ground at or near the surface, and the electrical energy is converted to seismic energy by the interaction of underground fluids, including hydrocarbons, with the rock matrix. The seismic waves are detected at or near the surface by seismic receivers.
In electroseismic exploration, it is generally impractical to deliver to the ground a single pulse containing enough electrical energy to produce an acceptable seismic return. Therefore, in electroseismic prospecting, the input electrical signal should preferably be a controlled wavetrain of predetermined length. A similar problem exists in conventional seismic exploration when a seismic vibrator is used as the seismic source instead of an explosive device. The seismic vibrator generates a controlled wavetrain (known as a sweep) which is injected into the earth. This wavetrain reflects from subsurface reflectors and the reflected wavetrain is recorded by seismic detectors located at or near the surface of the earth. The recorded wavetrain represents the input wavetrain convolved with the impulse response of the earth. In order to consolidate the seismic energy in the recorded wavetrain, and to observe underground reflection events relative to a time zero in the manner afforded by a single explosion source, a data processing step is employed in which the recorded seismic data are correlated with a reference wavetrain. Persons skilled in the art will understand the process of correlating two waves. (See, for example, Seismic Data Processing, O. Yilmaz, Society of Exploration Geophysicists (1987), pp. 18-19.) Electroseismic data are also processed using a similar correlation step.
It is known that the reference waveform used for the correlation should resemble the waveform of the expected seismic return. In the case of conventional seismic, the seismic response is linear, i.e., the output signal is proportional to the input signal to the first power. Hence the vibrator sweep wavetrain itself is a preferable reference waveform to use to correlate vibrator data. Electroseismic conversion also occurs as a linear process in which case the preferable reference waveform for correlation is often the source waveform. Selection of source waveforms and associated reference waveforms for linear electroseismic exploration is the subject of U.S. patent application Ser. No. 09/809,472 by Hornbostel and Thompson, published Sep. 27, 2001 with publication number WO 01/71386.
When a source waveform is correlated with its associated reference, a large peak will typically result at the onset time of the waveform surrounded by lower peaks at earlier and later times. (See patent publication WO 01/71386). These lower peaks are called correlation side lobes. They are undesirable because they provide no additional information and they can mask smaller desired returns.
An effective input current source for electroseismic exploration must have the following characteristics (see the aforementioned Patent Application):
The source should produce large current levels over extended time.
The source should have high electrical efficiency.
The source should contain little or no DC to avoid plating the electrode array.
The frequency content of the source should be adequate for the exploration needs.
The correlation of the source waveform with its reference should have sufficiently low side lobe levels.
Electroseismic prospecting holds great promise as a geophysical exploration tool. However, the utility of electroseismic prospecting may be enhanced by increasing the amount and types of information made available to an explorationist from an electroseismic prospecting operation. The present invention provides one method of doing so.
In some embodiments, the present invention is a method for nonlinear electroseismic prospecting comprising the steps of (a) selecting a source waveform of predetermined length, (b) generating an electrical signal based on the source waveform, (c) transmitting the electrical signal into the ground, (d) detecting and recording the seismic signals resulting from conversion of the electrical signal to seismic energy in subterranean formations, and (e) correlating the resulting seismic signals with a reference waveform to produce a correlated seismic record that simulates the result that would be produced by a single-pulse source aggregating all of the energy of the input sweep. The reference waveform is selected in conjunction with the source waveform to satisfy at least two criteria: (1) when the square of the source waveform (representing the nonlinear electroseismic return) is cross-correlated with the reference waveform, side lobes are reduced in amplitude to an acceptable level; and (2) when the source waveform is cross-correlated with the reference waveform (representing the linear electroseismic return), the resulting unwanted waveform""s interference with the desired correlation (the correlation of the square of the source waveform) is reduced to an acceptable level.
In some embodiments of the present invention, the source waveform is constructed from individual cycles of a 60-cycles/sec (Hz) sine wave, i.e., standard AC electrical power, with the polarity of some such cycles inverted as governed by a digital code. The digital code is the means by which the source wave is custom designed, using specific techniques taught herein, to satisfy the two above-enumerated criteria. Where deeper penetration of the subsurface is desired, another embodiment of the present invention constructs frequencies lower than 60 Hz by switching between the three phases of a 3-phase power source.
In preferred embodiments of the invention, the digital codes are sequences of ones, zeros, and/or negative ones, constructed according to the teachings of the invention with the starting point being a maximal length shift-register sequence. The length of such a sequence can be increased as a further means of side lobe reduction. Circular correlation with the corresponding, custom designed reference sequence is the preferred means of correlation in step (e) above for the above-described source sequences used in the preferred embodiments.
A person skilled in the art will often be able to examine the electroseismic results from the present inventive method and, from the seismic amplitudes, predict which regions contain hydrocarbons.
A preferred apparatus for generating the required waveforms is also disclosed.